You can use the same cable as long as you are using the rear serial connection.
The USB-1 won't work with the P2, and the regular usb cable from the P2 won't work with the XT.
That's only if you configure the rear port on the scanner first - it's a step a lot of folks starting off forget to do.
I suspect that the method for doing this on the P2 scanners is no different than the one found on the XT scanners - and we have that method defined in the FreeScan user guide...
As was mentioned, the cables are different, so rather than switching the cable at the scanner, switch it at the PC. Actually, you can just leave both connected and change the port setting between reading and writing.